Marked improvements in the performance of photographic emulsions began in the 1980's, resulting from the introduction of tabular grain emulsions into photographic products. A wide range of photographic advantages have been provided by tabular grain emulsions, such as improved speed-granularity relationships, increased covering power (both on an absolute basis and as a function of binder hardening), more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats.
Although tabular grain emulsions can be selected to provide a variety of performance advantages, depending upon the photographic application to be served, initially commercial interest focused on achieving the highest attainable photographic speeds with minimal attendant granularity. This capability of high bromide {111 } tabular grain emulsions was demonstrated by Wilgus et al U.S. Pat. No. 4,434,226 and Kofron et al U.S. Pat. No. 4,439,520. It was, of course, recognized that minor amounts of iodide further improve the speed-granularity relationship, and Solberg et al U.S. Pat. No. 4,433,048 taught that increased iodide concentrations near the peripheral edge of the tabular grains further improved the speed-granularity relationship.
Sometime after silver iodobromide {111} tabular grain emulsions appeared in photographic film products Ikeda et al U.S. Pat. No. 4,806,461 microscopically examined high bromide {111} tabular grains and concluded their superior speed-granularity could be attributed in part to the presence of 10 or more dislocations in tabular grains accounting for at least 50 percent of total grain projected area. This observation was reiterated by Takehara et al U.S. Pat. No. 5,068,173, Haga et al U.S. Pat. No. 5,472,836, Suga et al U.S. Pat. No. 5,550,012, and Maruyama et al U.S. Pat. No. 5,550,014. Before tabular grain emulsions were commercially used and before the observations of Ikeda et al, dislocations had been observed in high bromide {111} tabular grains and postulated to facilitate latent image formation, as illustrated by G. C. Farnell, R. B. Flint and J. B. Chanter, "Preferred Sites for Latent-Image Formation", J. Photogr. Sci., 13:25(1965); J. F. Hamilton, "Electron-Microscope Study of Defect Structure and Photolysis in Silver Bromide Microcrystals", Photogr. Sci. Eng., 11:57(1967); and G. C. Farnell, R. L. Jenkins and L. R. Solman, "Grain Disorder and its Influence on Emulsion Response", J. Photogr. Sci., 24:1(1976).
Black et al U.S. Pat. No. 5,709,988 discovered that the crystal lattice dislocations in the central region of high bromide {111} tabular grains increase pressure sensitivity while the crystal lattice dislocations in the peripheral region of the tabular grains increase sensitivity without increasing pressure sensitivity.
The first high chloride tabular grain emulsions contained {111} tabular grains, as illustrated by Wey U.S. Pat. No. 4,399,215 and Maskasky U.S. Pat. No. 4,400,463. Maskasky U.S. Pat. Nos. 5,292,632 and 5,275,930 overcame the problem of high chloride {111} tabular grain morphological instability by providing the first high chloride {100} tabular grain emulsions. The combination of the known ecological and developability advantages of high chloride emulsions with the higher tabular shape stability of high chloride grains with {100} crystal faces has stimulated interest in high chloride {100} tabular grain emulsions.
Several forms of incorporation of iodide into high chloride {100} tabular grains have been investigated. House et al U.S. Pat. No. 5,320,938, Chang et al U.S. Pat. No. 5,413,904, and Saito EPO 0 670 515 realized advantages to be obtained when iodide is added at or near grain nucleation.
Brust et al U.S. Pat. No. 5,314,798 observed a speed increase without an offsetting granularity increase when a band of higher iodide content is grown onto a high chloride {100} tabular grain. Potassium iodide is the demonstrated source of iodide during band formation. Chang et al U.S. Pat. No. 5,663,041 is cumulative with Brust et al.
Maskasky, Scaccia and Chen U.S. Pat. No. 5,792,602 discloses a process of preparing iodohalide grains wherein iodide ion is introduced into the crystal lattice structure by introducing elemental iodine into the dispersing medium and maintaining the dispersing medium within a pH range of from 5 to 8.